1. Field of the Invention
The present invention relates to a toner concentration controlling apparatus for controlling the toner concentration of a two-component developer for use in an electrophotographic printer for example.
2. Description of the Related Art
As a demand increases for a high-speed computer system operating with high-reliability, a printer capable of printing at a high speed while also providing good printing results is also increasingly demanded. Such a high-speed printer generally uses a developer consisting of a plurality of components including toner. For example, use may be made of a two-component developer consisting of toner and carrier (magnetic particles).
In a printer using such a two-component developer, the toner concentration of the developer stored in a developer unit decreases as the printing operation proceeds, so that an additional dose of the toner need be supplemented to the developer unit. The toner supply may be performed under the control of a toner concentration controlling apparatus.
A toner concentration controlling apparatus typically incorporates a sensor for detecting the toner concentration of a developer. When the sensor outputs a detection voltage which exceeds a predetermined threshold value, it is determined that the toner concentration has dropped to a level which requires additional toner supply. Thus, an additional amount of the toner is supplied to the developer unit.
With such a toner concentration controlling apparatus, the toner is supplied regardless of whether the printer is in a print-on state or in a print-off state in performing intermittent printing. However, immediately after the toner is supplied to the developer unit, the additional dose of the toner is not sufficiently mixed with the developer remaining in the developer unit, so that an excessive amount of the toner may adhere onto a recording paper. Here, by the term xe2x80x9cintermittent printingxe2x80x9d is meant any mode of printing (other than a continuous job of printing a large number of pages) where a certain pause exists between jobs of printing.
It is therefore an object of the present invention to provide a toner concentration controlling apparatus which is capable of reducing the likelihood that a dose of the toner is supplied in a print-on state in performing intermittent printing.
In accordance with the present invention, there is provided a concentration controlling apparatus for controlling toner concentration of a developer contained in a developer unit, the developer comprising a plurality of components including toner to be transferred to a photosensitive body for forming an electrostatic latent image. The concentration controlling apparatus comprises a toner concentration detector for detecting the toner concentration of the developer, a supply determiner for determining a need for toner supply in response to detection output from the toner concentration detector, and a toner supplier for supplying a dose of the toner to the developer unit in accordance with the determination by the supply determiner. The supply determiner determines the need for toner supply at a higher toner concentration in a print-off state than in a print-on state.
In a first embodiment, the supply determiner determines the need for toner supply by comparing the detection output with different threshold values in the print-on state and in the print-off state, respectively. Preferably, the supply determiner compares the detection output with a higher threshold value in the print-on state while comparing the detection output with a lower threshold value in the print-off state.
In a second embodiment, the supply determiner adjusts the detection output of the toner concentration detector produced in either one of the print-on state and the print-off state, and the supply determiner determines the need for toner supply by comparing the adjusted detection output with a common threshold value in said one of the print-on state and the print-off state while comparing the non-adjusted detection output with the common threshold value in the other of the print-on state and the print-off state. Preferably, the supply determiner multiplies the detection output of the toner concentration detector by a constant of greater than one only in the print-off state, and the supply determiner compares the multiplied detection output with the common threshold value in the print-off state while comparing the non-multiplied detection output with the common threshold value in the print-on state.
In a third embodiment, the toner concentration detector is supplied with a voltage to produce a detection output which varies in accordance with the voltage applied to the toner concentration detector even if the toner concentration does not change, and the controller further comprises a voltage controller for applying different voltages to the toner concentrating detector in the print-on state and in the print-off state, respectively. Preferably, the toner concentration detector is supplied with a higher voltage in the print-off state than in the print-on state.
In a fourth embodiment, the toner concentration controlling apparatus also comprises an agitator provided in the developer unit for agitating the developer, wherein the detection output of the toner concentration detector varies in accordance with the agitating speed of the agitating device. The apparatus further comprises an agitating speed controller for providing different agitating speeds of the agitator in the print-on state and in the print-off state, respectively.
In the fourth embodiment, the agitating speed controller preferably causes the agitator to agitate at a lower speed in the print-off state than in the print-on state. In this case, the toner concentration controlling apparatus may further comprise a development bias controller for providing a higher development bias in the print-off state than in the print-on state.
Other objects, features and advantages of the present invention will be apparent from the following description of the preferred embodiments given with reference to the accompanying drawings.